(1) Field of the Invention
The present invention relates to a filter using piezoelectric resonators, and more particularly relates to a filter module formed on a substrate used for a high-frequency circuit of a mobile communication terminal, such as a cellular phone or a wireless local access network (LAN).
(2) Description of Related Art
There have been demands for not only reduction in the size and weight of components built into mobile communication terminals, such as cellular phones or wireless LANs, but also an increase in the performance thereof. Therefore, high-performance and thin filters have been demanded. For the purpose of satisfying such demands, filters using piezoelectric resonators of a piezoelectric material have been suggested (see, for example, Japanese Patent No. 2800905 and U.S. Pat. No. 5,373,268). High-performance and thin filters may be able to be obtained by using, as such piezoelectric resonators, resonators utilizing longitudinal vibration along the thickness of the piezoelectric material, such as thin film bulk acoustic resonators (FBAR).
In order to reduce the sizes of such filters and increase the performance thereof, a plurality of piezoelectric resonators must be formed on one substrate. In order to provide filters that can be used at a high frequency, individual piezoelectric resonators need be improved in their characteristics, and, in addition, the arrangement of piezoelectric resonators, interconnects between piezoelectric resonators, and also connection between filters and external circuits need be optimized.
A technique in which a plurality of piezoelectric resonators are fabricated on one substrate is disclosed in Japanese Unexamined Patent Publication No. 9-64683. According to this publication, a plurality of piezoelectric resonators are formed on a single substrate or wafer and electrically connected through electrical connections patterned on the substrate or wafer, thereby reducing the size of a filter.